Masters of the Air

Don Powers' hummingbird research lands in the journal Nature

Using lasers, olive oil, a wind tunnel, and advanced imaging technology designed originally for engineers, biology professor Don Powers and his colleagues solved a mystery last spring. Their research, in short, redefines how hummingbirds fly. The scientists — Powers, Douglas Warrick from Oregon State University, and Bret Tobalske of the University of Portland — measured hummingbirds’ flight patterns last May. Their data disproved the prevailing hypothesis that hummingbirds’ flight is more akin to insects than birds. The results were published in the June 23, 2005, issue of Nature, the preeminent international journal of science.

Scientific breakthrough

For decades, the scientific community hypothesized that hummingbirds flew more like insects than birds because of their hovering ability and similar wing motion. “Our paper clearly shows this is not the case. Our research will rewrite the textbooks,” says Powers, chair of the Department of Biology and Chemistry.

In reality, when it comes to staying airborne, hummingbirds’ flight is halfway between birds and insects. Birds get all their lift from the downstroke of their wings, and insects get equal amounts of lift from both the downstroke and upstroke. Hummingbirds, by contrast, get about 75 percent of their lift from their wings’ downstroke and the remaining 25 percent from the upstroke.

“This discovery is the pinnacle for me professionally,” says Powers, who is one of only a handful of researchers in the country dedicated to the study of hummingbirds. “I don’t know what else we could discover about hummingbird flight that will be as big as this.”

To track the birds’ hovering mechanics, Powers and his colleagues used a sophisticated imaging technique, digital particle imaging velocimetry. While the hummingbirds hovered in a wind tunnel misted with microscopic droplets of olive oil, the researchers shot a computer-equipped laser at the birds. As the wing beats caused the air to circulate — and the oil particles to move — digital images were taken millionths of a second apart, allowing the scientists to record the timing, location, and intensity of lift production.

Previous experiments on how the birds stayed aloft relied on high-speed video equipment that could not fully reveal the underlying aerodynamics.

Like insects, hummingbirds can invert their wings, turning them upside down during their upstroke. But hummingbird wings have bones and feathers, so there are limitations on how far they can move structurally toward an insect model, Powers says.

“For years, the dogma had been that when hummingbirds inverted their wings they continued to produce lift,” says Powers, who earned a doctorate in physiological ecology at the University of California, Davis, in 1989. “There was also circumstantial evidence that suggested their up and down strokes were symmetrical. If you were to read a book on hummingbird hovering, that’s what it would tell you.”

The new research findings may reverberate beyond avian science. Igor Sikorsky, inventor of the helicopter, considered the flight of hummingbirds in the design process of his rotary-winged aircraft. It would not be surprising, then, if this information inspires new ways to think about the process of flight.

At the same time, Powers can’t help but see the humor in the study’s dependence on olive oil — “something you go down to Safeway to buy,” he chuckles.

Chance encounter

Powers originally planned to study marine invertebrates when he began pursuit of a master’s degree in biology at San Diego State in 1979. But upon arrival, Powers discovered that “everyone was studying the subject,” as he puts it, and he wanted to do something different.

What did interest him was avian science — a fascination first ignited during bird walks in his native Southern California when he was young. “I still approach science like a child,” he comments. “If I didn’t, it would just be a job.”

At the suggestion of his advisor, Powers visited the library to research birds. The first thing he pulled from the shelf was an ecology journal featuring hummingbirds. He didn’t need to read more; he knew right then what he would specialize in.

“It was one of those fortunate decisions because if I had decided to study chickens or pigeons, I would not have the same professional stature that I have now,” Powers says. “There are a lot of people who study chickens and pigeons. Only a handful have made a career out of studying hummingbird biology.”

As one colleague told him, Powers has put George Fox on the map in the field of physiological ecology. He has published more than 20 papers on the birds and makes regular pilgrimages to feeding grounds in Arizona, where he conducts field studies on everything from the birds’ energy-conservation strategy to this summer’s evaluation of the energy costs of hovering. To collect data for this study, Powers set up a feeder equipped with a respiratory mask, allowing him to measure the birds’ metabolic rate as they hovered.

Don Powers and his colleagues placed hummingbirds in a wind tunnel seeded with droplets of olive oil, which were illuminated with pulses of light, to study their flight (below). He evaluates the birds’ metabolic rate by equipping feeders with respiratory masks (left).

Unending quest

Ultimately, Powers is intrigued by the birds’ physiology. Of special interest is the hummingbirds’ use of torpor, a hibernation-like state that helps them conserve energy. The same bird with a heart rate of 1,200 beats per minute during exercise can slow the pace to fewer than 10 beats a minute at night.

While other birds use the tactic, hummingbirds’ use of it is perhaps the most pronounced. Powers’ lab studies have revealed that hummingbirds with easy access to food during the day often don’t go into torpor at night. Conversely, birds that have to fight for food — those that expend more energy and, consequently, don’t store as much fat — use torpor more frequently.

There is one dilemma with torpor studies: In the wild, the birds are impossible to find at night. “The way to solve that problem would be to use radio transmitters, but they haven’t made something small enough to put on a hummingbird yet,” Powers says. “If they ever do that, there’s going to be an explosion of papers on hummingbird torpor.”

It’s that element — the many unanswered questions — that drives him. “I tell my students that every time you walk through a door, there are 10 more waiting to be unlocked,” says Powers, who teaches courses in ornithology, ecology, animal physiology, and non-majors biology.

“There’s a lot I don’t know about hummingbirds. Just look at the whole thing with hovering flight. Everyone thought they had it all figured out. Then, in three weeks of data collection, we undid all that. Technology has allowed us to dive into areas that couldn’t be explored before.”

Questions remain about the birds’ mating rituals, migration patterns, eating habits, and water regulation. Of particular intrigue to Powers is the fact these tiny birds travel great distances every year: The rufous, the most common species in Oregon, migrates from Mexico to as far north as Alaska and back again.

How do they find their way? How do they store enough energy for the migration?

Amid the questions, one thing is certain — hummingbirds fill an ecological niche: As nectarivores — animals that feed on flower nectar — they serve as important flower pollinators in the Western Hemisphere. In the Eastern Hemisphere, the sunbird, with the head of a hummingbird and body of a sparrow, is one of a few other bird groups that fill that role.

The development of hummingbirds in the Western Hemisphere points to natural selection, the adaptation of organisms to adjust to a particular environment, Powers says. “There were plants that needed to be pollinated and birds that needed a place to exploit for food while minimizing competition and, voila, a hummingbird developed out of that.”

Such an intricate process points to the brilliance of the creator. “There is nothing that illustrates intelligent design better than the construction of organisms that are capable of shaping themselves to function successfully in complex environments that are also ever-changing,” he says. GFJ

Hummingbirds: Fast Facts

Hummingbirds are the only birds that truly hover and fly backwards.

The smallest weighs less than a penny.

Some can flap their wings 60 times in one second.

Their heart rates can race more than 1,200 beats per minute.

Their fluid intake is the equivalent of a 200-pound human male drinking between
400 and 800 pounds of water a day.

Despite their small size, they are a warm-blooded animal.

They only exist in the Western Hemisphere, and there may be more than 350 species.

Their metabolic rate is among the highest measured in vertebrate animals.